It has hitherto been known in the art that the impact resistance of vinyl chloride resin products can be improved in each instance by blending withe the polyvinyl chloride resin a graft copolymer obtained by polymerizing one or more vinyl monomers such as styrene, acrylonitrile and methyl methacrylate in the presence of a rubbery material. The vinyl chloride resin composition thus obtained has fairly satisfactory transparency and impact resistance, but a formed product of the resin composition, when bent or folded, is liable to be stress-whitened around the portion at which the product is bent or folded. Especially, a formed resin product possessing high impact resistance tends to be stress-whitened notably whereby the commercial value of the product is impaired.
In general, it is difficult to obtain satisfactory impact resistance and stress-whitening resistance or anti-stress-whitening property at the same time. There has been an attempt to improve both these properties by suitably coagulating a rubber latex and graft-polymerising thereonto styrene, methyl methacrylate or the like as is disclosed in Japanese Patent Pub. No. 31462/1971 and Japanese Patent Pub. No. 1584/1979, but the suitable coagulation of the latex requires a high level of technology, and thus a simple method has been desired.
It has been known, on the other hand, that the stress-whitening resistance of vinyl chloride resin compositions can be improved by adding to the polyvinyl chloride (PVC) resin a graft copolymer comprising a relatively hard rubber component such as a highly crosslinked butadiene rubber having a low degree of swelling. However, the impact resistance tends to be lowered as the degree of crosslinkage increases, so that it has been difficult to attain both high impact resistance and improved stress-whitening resistance at the same time.